1. Field of the Invention
The present invention relates to a technique of driving an inductive load by use of semiconductor switching elements, and more particularly to an inductive load driving method in which an H-bridge is formed to drive an inductive load and an inductive load driving apparatus which realizes such a method.
2. Description of the Related Art
In general, a stepping motor is constructed such that it has a rotor including a rotatable moving magnet and a plurality of driving coils each including an electromagnet are arranged around the rotor. In the stepping motor, the position and the rotating speed of the rotor can be controlled in an open loop by selecting the driving coils to cause the flow of a pulse-like current having a predetermined amplitude. In recent years, therefore, the stepping motors have widely been used as convenient motors.
In general, the stepping motor is an inductive load. As methods for driving such a load, there have widely been used a uni-polar driving method in which a current is flown in a fixed direction and a bi-polar driving system in which a current can be flown in either a forward direction or a reverse direction. In order that a switching current having a constant amplitude is caused to flow through the inductive load, each driving method is such that when a current supplied from a power source becomes equal to or larger than a predetermined value, a current is caused to flow through the flywheel diode connected in reverse parallel to the semiconductor switching element to release an energy stored in the inductive load, thereby attenuating the current flowing through the inductive load.
An example of such an inductive load driving method is shown in FIG. 5.
In FIG. 5, reference numeral 102 denotes an inductive load driving apparatus according to the prior art in which an H-bridge circuit is formed by an inductive load 131 in a stepping motor and four transistors 111 to 114. The upper transistors 111 and 112 are connected to a power source 132, and the lower transistors 114 and 113 are connected to a ground potential through a current detecting resistor 133.
Flywheel diodes 121 and 122 are connected in reverse parallel to the upper transistors 111 and 112, respectively. Similarly, flywheel diodes 123 and 124 are respectively connected in reverse parallel to the lower transistors 113 and 114 across the transistors 113 and 114 and the current detecting resistor 133.
The base terminals of the transistors 111 to 114 are connected to a control circuit 134 so that the operation of the transistor is controlled by the control circuit 134. Now assume that the transistors 111 and 113 are in turned-on conditions while the transistors 112 and 114 are in turned-off conditions. In this state, a supply current is supplied from the power source 132 to the inductive load 131 in a direction indicated by reference numeral 141.
The supply current 141 flows through the current detecting resistor 133. When a voltage generated across the current detecting resistor 133 becomes larger than a reference voltage 136, the output of a comparator 135 is inverted and the control circuit 134 detects the inversion of the comparator output to stop the supply current 141, thereby attenuating the current flowing through inductive load 131. After the lapse of a predetermined time, the control circuit 134 causes the flow of the supply current 141 from the power source 132 to the inductive load 131 again and stops the supply current 141 in accordance with the inversion of an output signal of the comparator 135. With the repetition of such an operation, a switching current flowing through the inductive load 131 can maintain a predetermined level.
There are two kinds of methods in the case where the control circuit 134 stops the supply current 141 from the power source 132 to attenuate the current flowing through the inductive load 131.
In one method, all of the transistors 111 to 114 are brought into turned-off conditions. At this time, the flywheel diodes 124 and 122 are reversely biased owing to an electromotive force generated in the inductive load 131 to cause the flow of a regeneration current indicated by reference numeral 142 in FIG. 6A. The flow of the regeneration current 142 causes the charging of (an output condenser of) the power source 132 so that a current flowing through the inductive load 131 is attenuated. In this case, it is possible to effectively utilize an energy stored in the inductive load 131.
In the other method, one of the transistors 111 and 113 is turned off. Now assume that in a state in which the supply current 141 is flowing, the transistor 111 is turned off with the transistor 113 being kept as it was turned on. Then, the flywheel diode 124 is forwardly biased owing to a reverse electromotive force of the inductive load 131 to cause the flow of a commutation current indicated by reference numeral 143 in FIG. 6B. The flow of the commutation current 143 causes the generation of heat from the flywheel diode 124 and the transistor 113 so that an energy stored in the inductive load 131 is consumed to attenuate the current. In this case, it is not possible to utilize the energy stored in the inductive load 131.
Comparing the regeneration current 142 and the commutation current 143, the regeneration current 142 can make the quick attenuation of a current flowing through the inductive load 131 at the time of switching of the current flowing through the inductive load 131 whereas the commutation current 143 can make the slow attenuation thereof.
However, in the case where the release of an energy stored in the inductive load 131 is tried in accordance with either one of the two methods mentioned above, a way based on the regeneration current 142 has a demerit that the attenuation is too rapid with the result that the ripple of the switching current flowing through the inductive load 131 is too large. On the other hand, a way based on the commutation current 143 has a demerit that the attenuation is too gentle with the result that the followability in changing a switching current level flowing through the inductive load 131 is poor. Also, when the driving by a switching current is tried in the case where there are a plurality of above-mentioned inductive loads 131 as in a two-phase stepping motor, there is a problem that in the case where frequencies for controlling respective currents flowing through the plurality of inductive loads are close to each other, beats are generated with the result that noises or vibrations become large.